Multilayered oscillating functional surface

ABSTRACT

A multilayered torsional hinged scanning mirror including a hinge layer with a mirror attaching member pivotally supported by torsional hinges. A mirror layer is bonded to the front side of the attaching member and a back layer is bonded to the back side of the attaching member. The mirror layer and the back layer are equal in mass and weight to balance the moment of inertia and stresses on the torsional hinges. The back layer may be a permanent magnet if the mirror oscillating drive is a magnetic drive. Alternately, the back layer may be another silicon slice.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/424,915 filed on Nov. 8, 2002, entitled “CompositeMEMS Micromirror Structure for High Frequency Operation Without DynamicDeformation,” which application is hereby incorporated herein byreference.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application relates to the co-pending and commonly assignedpatent application Ser. No. ______, entitled “Multilayered OscillatingDevice with Spine Support,” (Attorney Docket TI-36490) filedconcurrently herewith, which application is hereby incorporated hereinby reference.

TECHNICAL FIELD

[0003] The present invention relates generally to rapidly movingfunctional surfaces, such as mirrors, and “scanning mirrors.” Morespecifically, the invention relates to multilayered MEMS (micro-electricmechanical systems) torsional hinge functional surfaces, such asmirrors, operating at the resonance frequency of the device. A hingelayer having a first set of torsional hinges for providing the back andforth pivoting at a controlled frequency about a first axis includes anattaching member with a front side and a back side. A functional surfacelayer, such as a mirror, having a reflective surface is bonded ormounted to the front side of the hinge layer, and a back layer having amass moment equal to the mass moment of the functional surface layer isbonded or mounted to the back side of the hinge layer. According to oneembodiment, the mass moment of the front layer is the mass of the frontlayer times the distance or offset of the center of the mass of thefront layer from the first axis and the mass moment of the back layer isthe mass of the back layer times the distance or offset of the center ofthe mass of the back layer from the first axis.

[0004] According to one embodiment, the hinge layer further comprises asecond pair of torsional hinges for rapidly pivoting the functionalsurface of the device about a second axis to control movement in adirection substantially orthogonal to the pivoting movement about thefirst set of torsional hinges. When the functional surface of the deviceis a mirror, such mirrors are particularly suited for use as the driveengine for a laser printer and for generating a display on a screen.However, such mirrors may also be used to provide rapid switching in afiber optic communication system.

BACKGROUND

[0005] Rotating polygon scanning mirrors are typically used in laserprinters to provide a “raster” scan of the image of a laser light sourceacross a moving photosensitive medium, such as a rotating drum. Such asystem requires that the rotation of the photosensitive drum and therotating polygon mirror be synchronized so that the beam of light (laserbeam) sweeps or scans across the rotating drum in one direction as afacet of the polygon mirror rotates past the laser beam. The next facetof the rotating polygon mirror generates a similar scan or sweep whichalso traverses the rotating photosensitive drum but provides an imageline that is spaced or displaced from the previous image line.

[0006] There have also been prior art efforts to use a less expensiveflat mirror with a single reflective surface to provide a scanning beam.For example, a dual axis or single axis scanning mirror may be used togenerate the beam sweep or scan instead of a rotating polygon mirror.The rotating photosensitive drum and the scanning mirror aresynchronized as the drum rotates in a forward direction to produce aprinted image line on the medium that is at right angles or orthogonalwith the beam scan or sweep generated by the pivoting mirror.

[0007] However, with single axis mirrors the return sweep will traversea trajectory on the moving photosensitive drum that is at an angle withthe printed image line resulting from the previous or forward sweep.Consequently, use of a single axis resonant mirror, according to theprior art, required that the modulation of the reflected light beam beinterrupted as the mirror completed the return sweep or cycle, and thenturned on again as the beam starts scanning in the original direction.Using only one of the sweep directions of the mirror, of course, reducesthe print speed. Therefore, to effectively use an inexpensive scanningmirror to provide bi-directional printing, the prior art typicallyrequired that the beam scan move in a direction perpendicular to thescan such that the sweep of the mirror in each direction generatesimages on a moving or rotating photosensitive drum that are alwaysparallel. This continuous perpendicular adjustment is preferablyaccomplished by the use of a dual axis torsional mirror, but could beaccomplished by using a pair of single axis torsional mirrors. It hasbeen discovered, however, at today's high print speeds both forward andreverse sweeps of a single axis mirror may be used, and that noorthogonal adjustment is necessary.

[0008] Texas Instruments presently manufactures torsional dual axis andsingle axis resonant mirror MEMS devices fabricated out of a singlepiece of material (such as silicon, for example) typically having athickness of about 100-115 microns. The dual axis layout consists of amirror normally supported on a gimbal frame by two silicon torsionalhinges, whereas for a single axis mirror the mirror is supporteddirectly by a pair of torsional hinges. The reflective surface may be ofany desired shape, although an elliptical shape having a long axis ofabout 4.0 millimeters and a short axis of about 1.5 millimeters isparticularly useful. The elongated ellipse-shaped mirror is matched tothe shape that the angle of the beam is received. The gimbal frame usedby the dual axis mirror is attached to a support frame by another set oftorsional hinges. These mirrors manufactured by Texas Instruments areparticularly suitable for use as the scanning engine for high-speedlaser printers and visual displays. These high-speed mirrors are alsosuitable for use as high-speed optical switches in communicationsystems. One example of a dual axis torsional hinged mirror is disclosedin U.S. Pat. No. 6,295,154 entitled “Optical Switching Apparatus” andwas assigned to the same assignee on the present invention.

[0009] The present invention is particularly applicable to a mirror orreflective surface supported by torsional hinges and the discussion andembodiments are primarily with respect to mirrors. However, as suggestedby the title and the above discussion, the invention is also applicableto “functional surfaces” other than mirrors that have a need forhigh-speed pivoting or oscillations. Therefore, functional surfacesother than mirrors may include light gratings as well as surfaces notconcerned with light beams and the movement of light beams.

[0010] Therefore, it will be appreciated that, although many referencesand embodiment in the specification are with respect to mirrors, theclaims are not to be so limited except for such specific limitations inthe claims.

[0011] According to the prior art, torsional hinge devices wereinitially driven directly by magnetic coils interacting with smallmagnets mounted on the pivoting device at a location orthogonal to andaway from the pivoting axis to oscillate the device or, in the case of amirror functional surface, create the sweeping movement of the beam. Ina similar manner, orthogonal movement of a beam sweep was alsocontrolled by magnetic coils interacting with magnets mounted on thegimbals frame at a location orthogonal to the axis used to pivot thegimbals frame.

[0012] According to the earlier prior art, the magnetic coilscontrolling the functional surface or reflective surface portion of amirror typically received an alternating positive and negative signal ata frequency suitable for oscillating the device at the desired rate.Little or no consideration was given to the resonant pivoting frequencyof the device. Consequently, depending on the desired oscillatingfrequency or rate and the natural resonant frequency of the device aboutthe pair of torsional hinges, significant energy could be required topivot the device and especially to maintain the functional surface ofthe device in a state of oscillation. Furthermore, the magnets mountedon the functional surface portion added mass and limited the oscillatingspeed.

[0013] Later torsional devices, such as mirrors, were manufactured tohave a specific resonant frequency substantially equivalent to thedesired oscillation rate. Such resonant frequency devices wereparticularly useful when the functional surface of the devices was amirror used as a scanning engine. Various inertially coupled drivetechniques including the use of piezoelectric devices and electrostaticdevices have been used to initiate and keep the functional surface ormirror oscillations at the resonant frequency.

[0014] It has now been discovered that the earlier inexpensive anddependable magnetic drive can also be used and set up in such a way toboth maintain the pivoting device at its resonant frequency and toprovide orthogonal movement. Unfortunately, the added mass of themagnets becomes more and more of a problem as the required frequencyincreases to meet the higher and higher speed demands. Further, thefunctional surface of a device can be of almost any shape, includingsquare, round, elliptical, etc. However, an elongated elliptical shapehas been found to be particularly suitable if the functional surface isa mirror. Unfortunately, these elongated elliptical-shaped mirrorsintroduce moment of inertia forces that result in excess flexing andbending of the mirror adjacent the hinges and tips of the mirror suchthat the mirror no longer meets the required “flatness” specificationsfor providing a satisfactory laser beam. The thickness of the mirror maybe increased to maintain the necessary flatness, but the added weightand mass results in excess stress on the torsional hinges which cancause failures and/or reduced life.

[0015] Therefore, a scanning device having sufficient stiffness tomaintain acceptable flatness at high oscillation speeds would beadvantageous.

SUMMARY OF THE INVENTION

[0016] The problems mentioned above are addressed by the presentinvention, which provides a multilayered pivoting or oscillating device.Pivoting or oscillating mirror embodiments of the invention may be usedas the means of generating a sweeping or scanning beam of light across aphotosensitive medium. The device comprises a hinge layer that definesan attaching member pivotally supported along a first axis by a firstpair of torsional hinges extending to a support structure. The hingelayer has a front side and a back side. A functional layer is bonded tothe front side of the attaching member. For example, according to oneembodiment, a mirror layer having a reflection portion is bonded to thefront side of the attaching member, and a back layer having a massmoment (of back layer mass times back layer mass offset or distance fromthe first axis) substantially equal to the mass moment (mass of mirrorlayer times mirror layer mass offset or distance from the first axis) ofthe mirror layer (or other functional layer) is bonded to the back sideof the attaching member. The back layer may be a permanent magnet if thepivoting drive is a magnetic drive. Alternately, the back layer may beanother material, such as silicon, if the drive is an inertia coupleddrive.

[0017] According to another embodiment, the hinge layer comprises asupport member connected directly to the functional layer by the firstpair of torsional hinges. Alternately, according to a dual axisembodiment, the hinge layer includes a second pair of torsional hingesextending between a support member and a gimbals portion arranged toallow the gimbals portion to pivot about a second axis substantiallyorthogonal to the first axis. The reflective surface portion is attachedto the gimbals portion by the first pair of torsional hinges. When thefunctional surface of the device is a mirror, pivoting of the mirroralong the first axis and about the first pair of torsional hingesresults in a beam of light reflected from the reflective surfacesweeping back and forth, and pivoting about the second pair of torsionalhinges results in the reflected light moving substantially orthogonal tothe sweeping beam of light.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Other objects and advantages of the invention will becomeapparent upon reading the following detailed description and uponreferencing the accompanying drawings in which:

[0019]FIGS. 1A, 1B, and 1C illustrate the use of a rotating polygonmirror for generating the sweep of a laser printer according to theprior art;

[0020]FIGS. 2A and 2B are embodiments of a single axis scanningtorsional hinge device wherein the functional surface is a mirror;

[0021]FIGS. 3A, 3B, 3C, and 3D illustrate a prior art example of using asingle axis flat scanning mirror to generate a unidirectional beam sweepof a laser printer;

[0022]FIG. 4 is a perspective illustration of the single axis mirror ofthe type shown in FIG. 2A or 2B to generate the beam sweep of a laserprinter;

[0023]FIGS. 5A and 5B are perspective views of two embodiments of priorart two-axis torsional hinge devices (such as mirrors) for generating abi-directional movement of the functional surface;

[0024]FIGS. 6A, 6B, and 6C illustrate the use of one two-axis scanningmirror such as is shown in FIGS. 5A and 5B to generate a bi directionalbeam sweep of a laser;

[0025]FIG. 7 illustrates one embodiment of a single axis magnetic drive;

[0026]FIGS. 8A and 8B show an exploded view and an assembled view of amagnetic drive multilayered scanning device according to the presentinvention when the functional surface is a scanning mirror;

[0027]FIG. 9 illustrates a single axis magnetic drive according toanother embodiment;

[0028]FIGS. 10A, 10B and 10C illustrate the operation of a piezoelectricdrive to create inertia coupled oscillations in a scanning functionalsurface;

[0029]FIGS. 11A and 11B show an exploded view and an assembled view of apiezoelectric driven multilayered device of the present invention;

[0030]FIGS. 12A and 12B show an exploded view and an assembled view of amagnetic driven dual axis multilayered high-speed device when thefunctional surface is a mirror; and

[0031]FIGS. 13A and 13B show an exploded view and an assembled view of apiezoelectric driven dual axis multilayered high-speed device when thefunctional surface is a mirror.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0032] Like reference numbers in the figures are used herein todesignate like elements throughout the various views of the presentinvention. The figures are not intended to be drawn to scale and in someinstances, for illustrative purposes, the drawings may intentionally notbe to scale. One of ordinary skill in the art will appreciate the manypossible applications and variations of the present invention based onthe following examples of possible embodiments of the present invention.The present invention relates to a high-speed pivoting device, such as amirror, with a moveable reflecting surface that is suitable forproviding the raster scans for laser printers and displays or high-speedoptical switching. More specifically, the invention relates to apivoting device and a magnetic drive for maintaining high speed resonantpivoting of the functional surface about a pair of torsional hinges.

[0033] Referring now to FIGS. 1A, 1B and 1C, there is shown anillustration of the operation of a prior art printer using a rotatingpolygon mirror. As shown in FIG. 1A, there is a rotating polygon mirror10 which in the illustration has eight reflective surfaces 10 a-10 h. Alight source 12 produces a beam of light 14 a, such as a laser beam,that is focused on the rotating polygon mirror so that the beam of lightfrom the light source 12 is intercepted by the facets 10 a-10 h ofrotating polygon mirror 10. Thus, the laser beam of light 14 a from thelight source 12 is reflected from the facets 10 a-10 h of the polygonmirror 10 as illustrated by dashed line 14 b to a moving photosensitivemedium 16 such as a rotating photosensitive drum 18 having an axis ofrotation 20. The moving photosensitive medium 16 or drum 18 rotatesaround axis 20 in a direction as indicated by the arcurate arrow 22 suchthat the area of the moving photosensitive medium 16 or drum 18 exposedto the light beam 14 b is continuously changing. As shown in FIG. 1A,the polygon mirror 10 is also rotating about an axis 24 (axis isperpendicular to the drawing in this view) as indicated by the secondarcurate arrow 26. Thus, it can be seen that the leading edge 28 offacet 10 b of rotating polygon mirror 10 will be the first part of facet10 b to intercept the laser beam of light 14 a from the light source 12.As the mirror 10 rotates, each of the eight facets of mirror 10 willintercept the light beam 14 a in turn. As will be appreciated by thoseskilled in the art, the optics to focus the light beam, the lens systemto flatten the focal plane to the photosensitive drum, and any foldmirrors to change the direction of the scanned beam are omitted for easeof understanding.

[0034] Illustrated below the rotating polygon mirror 10 is a second viewof the photosensitive medium 16 or drum 18 as seen from the polygonscanner. As shown by the photosensitive drum view 18, there is thebeginning point 30 of an image of the laser beam 14 b on drum 18immediately after the facet 10 b intercepts the light beam 14 a andreflects it to the moving photosensitive medium 16 or drum 18.

[0035] Referring now to FIG. 1B, there is shown substantially the samearrangement as illustrated in FIG. 1A except the rotating polygon mirror10 has continued its rotation about axis 24 such that the facet 10 b hasrotated so that its interception of the laser beam 14 a is about to end.As will also be appreciated by those skilled in the art, because of thevarying angle the mirror facets present to the intercepted light beam 14a, the reflected light beam 14 b will move across the surface of therotating drum as shown by arrow 32 and dashed line 34 in FIG. 1B.

[0036] However, it will also be appreciated that since rotating drum 18was moving orthogonally with respect to the scanning movement of thelight beam 14 b, that if the axis of rotation 24 of the rotating mirrorwas exactly orthogonal to the axis 20 of the rotating photosensitivedrum 18, an image of the sweeping or scanning light beam on thephotosensitive drum would be recorded at a slight angle. As shown moreclearly by the lower view of the photosensitive drum 18, dashed line 34illustrates that the trajectory of the light beam 14 b is itself at aslight angle, whereas the solid line 36 representing the resulting imageon the photosensitive drum is not angled but orthogonal to the rotationor movement of the photosensitive medium 16. To accomplish this parallelprinted line image 36, the rotating axis 24 of the polygon mirror 10 istypically mounted at a slight tilt with respect to the rotatingphotosensitive drum 18 so that the amount of vertical travel or distancetraveled by the light beam along vertical axis 38 during a sweep or scanacross medium 16 is equal to the amount of movement or rotation of thephotosensitive medium 16 or drum 18. Alternately, if necessary, thistilt can also be accomplished using a fold mirror that is tilted.

[0037]FIG. 1C illustrates that facet 10 b of rotating polygon mirror 10has rotated away from the light beam 14 a, and facet 10 c has justintercepted the light beam. Thus, the process is repeated for a secondimage line. Continuous rotation will of course result in each facet ofrotating mirror 10 intercepting light beam 14 a so as to produce aseries of parallel and spaced image lines, such as image line 36 a,which when viewed together will form a line of print or other image.

[0038] It will be further appreciated by those skilled in the laserprinting art, that the rotating polygon mirror is a very precise andexpensive part or component of the laser printer that must spin atterrific speeds without undue wear of the bearings even for rather slowspeed printers. Therefore, it would be desirable if a less complex flatmirror, such as for example a resonant flat mirror, could be used toreplace the complex and heavy polygonal scanning mirror.

[0039]FIGS. 2A and 2B illustrate prior art single layer, single axistorsional devices where the functional surfaces are mirrors. Each of themirrors of FIGS. 2A and 2B include a support member 40 supporting amirror or reflective surface 42, which may be substantially any shapebut for many applications the elongated ellipse shape of FIG. 2B ispreferred. The functional surface, or mirror, is supported by a singlepair of torsional hinges 44 a and 44 b. Thus, it will be appreciatedthat, when the functional surface is a mirror, if the mirror portion 42can be maintained in an oscillation state around axis 46 by a drivesource, the mirror can be used to cause a sweeping light beam torepeatedly move across a photosensitive medium, or to rapidly switch alight beam to a selected one of a plurality of optical fibers.

[0040] It will also be appreciated that an alternate embodiment of asingle axis mirror or other functional surface may not require thesupport member or frame 40 as shown in FIGS. 2A and 2B. For example, asshown in both figures, the torsional hinges 44 a and 44 b may simplyextend to a pair of hinge anchor pads 48 a and 48 b as shown in dottedlines. The functional surface or, in the present example, mirror portion42, is suitably polished on its upper surface to provide a specular ormirror surface.

[0041] The prior art single layered devices were typically MEMS(micro-electric mechanical systems) type devices manufactured from aslice of single crystal silicon. Further, because of the advantageousmaterial properties of single crystalline silicon, such MEMS baseddevices have a very sharp torsional resonance. The Q of the torsionalresonance typically is in the range of 100 to over 1000. This sharpresonance results in a large mechanical amplification of the device'smotion at a resonance frequency versus a non-resonant frequency.Therefore, it is typically advantageous to pivot the device about thescanning axis at the resonant frequency. This dramatically reduces thepower needed to maintain the device in oscillation.

[0042] There are many possible drive mechanisms available to provide theoscillating movement about the scan axis. For example, FIG. 2Aillustrates a prior art magnetic drive device wherein the functionalsurface is a mirror having a pair of permanent magnets 50 a and 50 bmounted on tabs 52 a and 52 b respectively. The permanent magnets 50 aand 50 b interact with a pair of coils (not shown) located below themirror structure. The mirror mechanical motion in the scan axis istypically required to be greater than 15 degrees and may be as great as30 degrees. Resonant drive methods involve applying a small rotationalmotion at or near the resonant frequency of the functional surfacedirectly to the torsionally hinged device, or alternately motion at theresonant frequency may be applied to the whole silicon structure, whichthen excites the device to resonantly pivot or oscillate about itstorsional axis. In inertial resonant type of drive methods a very smallmotion of the whole silicon structure can excite a very large rotationalmotion of the device. Suitable inertial resonant drive sources includepiezoelectric drives and electrostatic drive circuits. A magneticresonant drive that applies a resonant magnetic force directly to thetorsional hinged functional surface portion has also been found to besuitable for generating the resonant oscillation for producing the backand forth beam sweep when the functional surface is a mirror.

[0043] Further, by carefully controlling the dimension of hinges 44 aand 44 b (i.e., width, length and thickness) the device may bemanufactured to have a natural resonant frequency which is substantiallythe same as the desired pivoting speed or oscillating frequency of thedevice. Thus, by providing a functional surface, such as a mirror, witha high-speed resonant frequency substantially equal to the desiredpivoting speed or oscillating frequency, the power loading may bereduced.

[0044] Referring now to FIGS. 3A, 3B, 3C and 3D, there is illustrated aprior art example of a laser printer using a single-axis oscillatingmirror to generate the beam sweep. As will be appreciated by thoseskilled in the art and as illustrated in the following figures, priorart efforts have typically been limited to only using one direction ofthe oscillating beam sweep because of the non-parallel image linesgenerated by the return sweep. As shown in FIGS. 3A, 3B, 3C and 3D, thearrangement is substantially the same as shown in FIGS. 1A, 1B and 1Cexcept that the rotating polygon mirror has been replaced with a singleoscillating flat mirror 54 that oscillates in both directions asindicated by double headed arcuate arrow 56. As was the case withrespect to FIG. 1A, FIG. 3A illustrates the beginning of a beam sweep atpoint 30 by the single axis mirror 54. Likewise, arrow 32 and dashedline 34 in FIG. 3B illustrate the direction of the beam sweep as mirror54 substantially completes its scan as it rotates in a direction asindicated by arrow 56 a. Referring to the lower view of thephotosensitive drum 18, according to this prior art embodiment, themirror 54 is mounted at a slight angle such that the beam sweep issynchronized with the movement of the rotating drum 18 so that thedistance the medium moves is equal to the vertical distance the lightbeam moves during a sweep. As was the case for the polygon mirror ofFIG. 1B, the slightly angled trajectory as illustrated by dashed line 34results in a horizontal image line 36 on the moving photosensitivemedium 16 or drum 18.

[0045] Thus, up to this point, it would appear that the flat surfacesingle torsional axis oscillating mirror 54 should work at least as wellas the rotating polygon mirror 10 as discussed with respect to FIGS. 1A,1B, and 1C. However, when the oscillating mirror starts pivoting back inthe opposite direction as shown by the arcuate arrow 56 b, with priorart scanning mirror printers, it was necessary to turn the beam,indicated by dashed line 34 a in FIG. 3C, off and not print during thereturn sweep since the vertical movement of the mirror resulting frombeing mounted at a slight angle and the movement of the movingphotosensitive medium 16 or rotating drum 18 were cumulative rather thansubtractive. Consequently, if used for printing, the angled trajectory34 a of the return beam combined with movement of the rotating drum 18would result in a printed image line 36 a which is at even a greaterangle than what would occur simply due to the movement of the rotatingphotosensitive drum 18. This, of course, is caused by the fact that asthe beam sweep returns, it will be moving in a downward direction asindicated by arrow 58 rather than an upward direction, whereas thephotosensitive drum movement is in the upward direction indicated byarrow 60. Thus, as stated above, the movement of the drum and the beamtrajectory are cumulative. Therefore, for satisfactory printing by aresonant scanning mirror printer according to the prior art, it wasunderstood that the light beam and the printing were typicallyinterrupted and/or stopped during the return trajectory of the scan.Thus, the oscillating mirror 54 was required to complete its reversescan and then start its forward scan again as indicated at 30A, at whichtime the modulated laser was again turned on and a second image lineprinted as indicated in FIG. 3D.

[0046]FIG. 4 illustrates a perspective illustration of a scanning mirrorused to generate an image on a medium 16. The mirror device 56, such asthe single axis mirror shown in FIGS. 2A and 2B, pivots about a singleaxis so that the reflecting surface 42 of the mirror device 56 receivesthe light beam 14 a from source 12 and provides the right to left andleft to right beam sweep 14 b between limits 64 and 66 as was discussedwith respect to FIGS. 3A, 3B, 3C and 3D. This left to right and right toleft beam sweep provides the parallel lines 68 and 70 as the medium 16moves in the direction indicated by arrow 72.

[0047] It will also be appreciated that various shapes of a functionalsurface can be used in the practice of this invention. Further, in thecase of a scanning mirror, the demand for higher and higher print speedswill require a higher and higher oscillation speed. Similarly,high-speed pivoting is also necessary when the functional surface is amirror used as high-speed optical switch. However, in addition tohigh-speed pivoting of the device, it is also important that thefunctional surface not deform as it pivots at high speed. Morespecifically, if the functional surface is a scanning mirror, it isimportant that the mirror not deform as it sweeps the laser beam acrossthe photosensitive medium during a scan cycle. One way to avoid flexingor deforming of the rapidly pivoting functional surface or mirror is toincrease the thickness of the functional surface. Unfortunately,increasing the device thickness results in increased stress on thetorsional hinges due to an increase in weight, mass and moment ofinertia.

[0048] Referring now to FIGS. 5A and 5B, there is shown a perspectiveview and a top view, respectively, of two bi-directional assemblieswherein the functional surfaces are mirrors. Such dual axis mirrors maybe used to provide a high-speed beam sweep wherein the high-speed beamsweep is also adjusted in a direction orthogonal to the beam sweep ofthe mirror. When used as a scanning engine for a printer, adjusting thebeam sweep orthogonally allows the printed image lines produced by abeam sweep in one direction and then in a reverse direction to bemaintained parallel to each other. As shown, the moveable assemblies ofboth FIGS. 5A and 5B are illustrated as being mounted on a support 74,and suitable for being driven along both axes 46 and 76. As wasdiscussed above with respect to single axis resonant devices, theassembly may be formed from a substantially planar material and thefunctional or moving parts may be etched in the planar sheet of material(such as silicon) by techniques similar to those used in semiconductorart. As shown, the functional components include a support member orframe portion 40, similar to the single axis device discussed above.However, unlike the single axis resonant device, the support structureof the dual axis device also includes an intermediate gimbals portion 78as well as the functional surface or mirror portion 42. It will beappreciated that the intermediate gimbals portion 78 is hinged to thesupport member or frame portion 40 at two ends by a pair of torsionalhinges 80 a and 80 b spaced apart and aligned along an axis 76. Exceptfor the pair of hinges 80 a and 80 b, the intermediate gimbals portion78 is separated from the frame portion 40. It should also be appreciatedthat, although support member or frame portion 40 provides an excellentsupport for attaching the device to support structure 74, it may bedesirable to eliminate the frame portion 40 and simply extend thetorsional hinges 80 a and 80 b and anchor the hinges directly to thesupport 74 as indicated by anchors 82 a and 82 b shown in dotted lineson FIGS. 5A and 5B.

[0049] The inner, centrally disposed functional surface, such as mirrorportion 42, is attached to gimbals portion 78 at hinges 44 a and 44 balong an axis 46 that is orthogonal to or rotated 90° from axis 76. Thereflective surface or mirror portion 42 is suitably polished on itsupper surface to provide a specular or mirror surface. If desired, acoating of suitable material can be placed on the mirror portion toenhance its reflectivity for specific radiation wavelengths.

[0050] As was mentioned above with respect to single axis devices, thereare many combinations of drive mechanisms for the scan or sweep axis.For the cross scan or orthogonal axis, since the angular motion requiredis usually much less, an electromagnetic drive may be used to produce acontrolled movement about the torsional hinges 80 a and 80 b toorthogonally move and position the beam sweep to a precise position.Consequently, a set of permanent magnet sets 84 a and 84 b may beassociated with the movement about hinges 80 a and 80 b.

[0051]FIGS. 6A, 6B and 6C illustrate the use of a dual axis scanningresonant mirror such as shown in FIGS. 5A and 5B as a scanning enginefor a laser printer. As can be seen from FIGS. 6A and 6B, the operationof a dual axis scanning mirror assembly 86 as it scans from right toleft in the figures is substantially the same as mirror 56 pivotingaround a single axis as discussed and shown in FIGS. 3A-3D. However,unlike the single axis mirror 56 and as shown in FIG. 6C, it is notrequired to turn the laser (light beam 14 b) off during the return scan,since a return or left to right scan in FIG. 6C can be continuouslymodulated during the return scan so as to produce a printed line orimage on the moving photosensitive medium 16. The second printed line ofimages, according to the present invention, will be parallel to theprevious right to left scan. This is, of course, accomplished by slightpivoting of the mirror 86 around orthogonal axis 76 of the dual axismirror as was discussed above.

[0052] Further, as was discussed above with respect to a single axismirror, by carefully controlling the dimension of hinges 44 a and 44 b(i.e., width, length and thickness) illustrated in FIGS. 5A and 5B, thedevice may be manufactured to have a natural resonant frequency which issubstantially the same as the desired oscillating frequency of thedevice. Thus, when the functional surface is a mirror, by providing themirror with a resonant frequency substantially equal to the desiredoscillating frequency, the power loading may be reduced.

[0053] From the above discussion, it will be appreciated that it isadvantageous to manufacture a scanning device having a mirror as thefunctional surface for use as a drive engine for a visual display orprinter and that the mirror have a resonant frequency substantially thesame as the desired raster or sweep frequency of the printer or display.As was also discussed, a magnetic drive is an inexpensive, dependableand effective technique for starting and maintaining the oscillatingfunctional surface, or mirror, at its resonant frequency. Unfortunately,the magnet sets 50 a and 50 b located on tabs 52 a and 52 b of thefunctional surface of FIG. 5A adds to the mass and moment of inertia ofthe resonant device, which in turn tends to reduce the resonantfrequency of the device. For example, the resonant frequency of one dualaxis magnetic drive device of the type shown in FIG. 5A is about 100 Hzand would be even lower if the size of the mirror or functional surfacewas increased. A speed of 100 Hz simply is not fast enough for many ifnot most mirror applications. Therefore a structure with a magneticdrive and increased resonant frequency would be advantageous.

[0054] Referring now to FIG. 7, there is a simplified illustration of apivoting functional surface 88 (such as the mirror shown in FIGS. 8A and8B) and a permanent magnet arrangement that significantly reduces theinertia forces of the apparatus. As shown in FIGS. 8A and 8B, the tabs52 a and 52 b of FIG. 5A used to mount the permanent magnet sets 50 aand 50 b have been eliminated and a single magnet 90 is mounted behindthe mirror 88. According to the embodiment shown in FIG. 7, magnet 90has a diametral charge perpendicular to the axis of rotation, asillustrated by double headed arrow 94, rather than an axial charge. Itwill, of course, also be necessary to relocate the drive coil 96 so thatit is substantially below magnet 90.

[0055]FIG. 9 shows a second magnetic drive arrangement. As shown, anaxial charged magnet 100 is used instead of the diametral charged magnetof FIG. 7. Further, the coil 96 shown in FIG. 7 is replaced by anelectro magnet device, such as device 102, having legs 104 a and 104 b,that extend to each side of the magnet 100. Thus, alternating currentapplied to coil 105 results in the magnetic field at the tips of legs104 a and 104 b continuously changing polarity. This change in polaritycreates alternating push-pull forces on magnet 100.

[0056] As also mentioned above, an inertially coupled resonant drivesystem may also be used to create resonant pivotal oscillation of thescanning device. FIGS. 10A, 10B and 10C illustrate an arrangement of aninertially coupled piezoelectric drive. FIGS. 10A and 10B show a topview and a side view respectively of a single axis torsional hingeddevice wherein a mirror is the functional surface. As shown, the mirroruses piezoelectric elements to drive a mirror of the type shown in FIGS.11A and 11B and to be discussed below, to resonance. As shown in FIGS.10A and 10B, the apparatus includes a support frame 106 having two longsides 108 a and 108 b and two short sides 110 a and 110 b. The shortside 110 a is mounted to support structure 112 by means of stand-off114. The mirror or functional surface portion 115 is attached to shortsides 110 a and 110 b by the torsional hinges 116 a and 116 b such thatthe oscillating portion 115 is located above a cavity 118 in supportstructure 112. Slices of piezoelectric material 120 a and 120 b arebonded to long sides 108 a and 108 b of the support frame 106 as shown.The slices of piezoelectric material are sliced so that they bend orcurve when a voltage is applied across the length of the strip or sliceof material. As will be appreciated by those skilled in the art ofpiezoelectric materials, the response time is extremely fast such thatan alternating voltage even having a frequency as high as between 2-25KH_(z) will cause the material to bend and flex at the same frequency asthe applied voltage. Therefore, since the slices of piezoelectricmaterial are bonded to the support frame of the device, the applicationof an alternating voltage through conductors or wires 122 a and 122 bfrom the AC voltage source 124 and having a frequency substantiallyequal to the resonant frequency of the oscillating portion 115 as shownin FIG. 10C will cause vibration motion to be inertially coupled to theoscillating portion 115 of the device and thereby initiate and maintainthe device in resonant pivoting oscillation.

[0057] The arrangement of piezoelectric slices discussed with respect toFIGS. 10A, 10B and 10C is for example only and other arrangements may beequally suitable for generating resonant motion.

[0058] Thus, from the above discussion it will be appreciate thatalthough all types of high-speed devices, such as scanning mirrors, maybe used in high-speed optical switches as well as various printer anddisplay applications, resonant scanning mirrors having elongatedelliptical shapes in the direction of rotation so that the light beamcan be reflected from the mirror surface as long as possible may be themost cost effective and suitable for use in high speed printers anddisplays. Further, mirrors used for high-speed optical switches arepreferably designed to have a resonant frequency that is equal to thedesired pivoting speed of the optical switches to help reduce powerloading. However, elongated functional surfaces, such as these elongatedelliptical shaped mirrors, introduce a new set of problems and concernswhen pivoting at high speed.

[0059] For example, such elongated elliptical mirrors are typicallymanufactured from a slice of single crystal silicon. At the same time,to achieve the very high resonant oscillation and hinge flexibilitynecessary to obtain sufficient rotational movement, it is necessary thatthe torsional hinges be very thin. Unfortunately, if the slice of singlecrystal silicon is sufficiently thin to fabricate torsional hinges thatoperate at high oscillating speeds, the structure may be too flexible touse as the reflecting surface of a mirror device. At high pivotingspeeds, the tips of an elongated elliptical mirror travel at very highspeeds and gain significant inertia. Consequently, the functionalsurface, or mirror, tends to flex excessively. This excessive flexing ofcourse means that during some portions of the oscillating cycle, thedevice bends or flexes and is not flat. This means, for many mirrorapplications the mirror has too much curvature or flex during theoscillating cycle. This variation in mirror flatness at high frequenciesis simply unacceptable for many displays, printers and optical switchingapplications.

[0060] One attempt at solving the conflict between the need for flexiblehinges and a rigid or flat reflecting surface is the use of anadditional layer of material to support the functional surface (such asfor example, a mirror). Therefore, referring again to FIGS. 8A and 8B,there is shown an exploded view and an assembled view of a single axismultilayered scanning device having a mirror as the functional surface.As shown, the multilayered scanning device comprises a support structureor hinge layer 126 for pivotally supporting an attaching member 128having a front side 130 and a back side 132 connected to an anchormember 134 by a pair of torsional hinges 136 a and 136 b. According tothis embodiment, anchor member 134 is a frame as shown. However,according to another embodiment, anchor member 134 could be replaced bya pair of anchor pads 138 a and 138 b as indicated by the dotted lines.The operational or functional portion 92 is typically thicker than thehinge layer 126 and has a front portion 140. When the functional surfaceis a mirror, the front portion 140 has a reflecting surface 142 and aback portion 144. The back portion 144 is bonded or mounted to the frontside 130 of the attaching member 128 and a back layer such as permanentmagnet 90 is bonded or mounted to the back side 132 of the attachingmember 128. As shown, permanent magnet 90 is bonded along the axis 146to the center of the back side 132 of attaching member 128. Permanentmagnet 90 is considerably stiffer than the hinge layer 124 andfunctional surface portion 92 and consequently stiffens and reinforcesthe structure in the middle area where the magnet is located. The massmoment of the permanent magnet 90 (mass of permanent magnet 90 times theoffset of the center of mass of the permanent magnet 90 from axis 146)is selected to be substantially equal to, and opposite the mass momentof the functional surface portion 92 (mass of functional surface portion92 times the offset of the center of mass of functional surface portion92 from axis 146) such that the moment of inertia of the assembledmultilayered torsional hinged device (or mirror) is centered on the axisof rotation extending through hinges 136 a and 136 b. More specifically,according to one embodiment of the invention, the mass moment of thefunctional surface or mirror layer is the product of the mass functionalsurface or mirror layer times the offset or distance of the center ofmass of the functional surface from the axis of rotation, and the massmoment of the back layer is the product of the mass of the back layertimes the offset or distance of the center of mass of the back layerfrom the axis of rotation. FIG. 8B shows the assembled structure. Thefunctional surface portion 92 and permanent magnet 90 of the assembledstructure of FIG. 8B, of course add significant weight that must besupported by the torsional hinges 136 a and 136 b. Therefore, to assurethat the hinges are not under excessive stress, the design of thetorsional hinges must consider the stress caused by the inertia forcesand the added weight to avoid unacceptable failure rates and short life.

[0061] In addition to an oscillating device having a magnetic drive,such as the mirror device shown in FIGS. 8A and 8B, the basic conceptsof the embodiments discussed with respect to these figures are alsoapplicable to resonant devices using inertia drive such as provided by apiezoelectric device as discussed above. Therefore, referring again toFIGS. 11A and 11B, there is shown an exploded view and an assembled viewof a multilayered resonant device suitable for use with a piezoelectricdrive. Those elements of the structure that are equivalent to theelements of FIGS. 8A and 8B carry the same reference numbers. Therefore,as shown, the embodiment of FIGS. 11A and 11B differs with respect toFIGS. 8A and 8B only in the presence of a back layer 156 made of amaterial such as silicon rather than the permanent magnet 90. However,as was the case for the embodiment of FIGS. 8A and 8B, the back layer156 is also selected to have a mass moment (mass of back layer 156 timesthe offset of the center of the mass of back layer 156 from axis 146)equal to the mass moment of the functional surface portion 92 (mass offunctional surface portion 92 times the offset of the center of the massfunctional surface portion 92 from axis 146).

[0062] Likewise, FIGS. 12A and 12B show a dual axis multilayeredmagnetic drive resonant device having a mirror as the functionalsurface. This embodiment is substantially the same as that discussedwith respect to FIGS. 8A and 8B except that the support structure orhinge layer 126 a further defines the gimbals portion 158 which pivotsorthogonally to the functional surface portion along torsional hinges160 a and 160 b and about axis 162.

[0063] Similarly, FIGS. 13A and 13B are similar to the embodiments shownin FIGS. 12A and 12B except that the support structure or hinge layer126 a further defines the gimbals portion 158, which pivots orthogonalto the functional surface along torsional hinges 160 a and 160 b andabout axis 162.

[0064] Each of the assembled dual axis devices illustrated by FIGS. 12Band 13B include a back portion (whether a permanent magnet 90 or pieceof silicon 156) that, once attached to the attaching member 132, has amass moment as defined above that is substantially equal to the massmoment of the functional surface portion 92 also defined above.Consequently, any moment of inertia of the device resulting from theseadded portions is centered on the two axes of rotation.

[0065] The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed as many modifications andvariations are possible in light of the above teaching. The embodimentswere chosen and described in order to best explain the principles of theinvention and its practical application to thereby enable others skilledin the art to best utilize the invention and various embodiments withvarious modifications as are suited to the particular use contemplated.It is intended that the scope of the invention be defined by the claimsappended hereto and their equivalents.

What is claimed is:
 1. A multilayered torsional hinged resonant pivotingdevice comprising: a hinge layer defining a support structure and anattaching member, said support structure for pivotally supporting saidattaching member along a first axis of rotation by a pair of torsionalhinges, and said attaching member having a front side and a back side; afront layer having a functional surface portion, and a back portion,said back portion of said front layer mounted to said front side of saidattaching member and said front layer having a known mass moment aboutsaid first axis; and a back layer mounted on said back side of saidattaching member and having a mass moment substantially equal to andopposite said known mass moment of said front layer, such that thecenter of mass of the combined front and back layers is substantiallycoplanar with the first axis of rotation and the moment of inertia ofsaid multilayered torsional hinged device is substantially centered onsaid first axis of rotation.
 2. The multilayered device of claim 1wherein said hinge layer comprises an anchor member connected to saidattaching member along said first axis by said first pair of torsionalhinges.
 3. The multilayered device of claim 2 wherein said anchor memberis a support frame.
 4. The multilayered device of claim 2 wherein saidanchor member is a pair of anchor pads.
 5. The multilayered device ofclaim 1 wherein said support structure of said hinge layer comprises agimbals portion connected to said attaching member along said first axisby said pair of torsional hinges and an anchor member pivotallysupporting said gimbals portion by a second pair of torsional hingesalong a second axis substantially orthogonal to said first axis.
 6. Themultilayered device of claim 5 wherein said anchor member is a supportframe.
 7. The multilayered device of claim 5 wherein said anchor memberis a pair of anchor pads.
 8. The multilayered device of claim 1 whereinsaid back layer is a permanent magnet.
 9. The multilayered device ofclaim 8 and further comprising a magnetic coil connected to analternating voltage having a frequency substantially equal to a selectedsweep frequency of said pivoting device and wherein said magnetic coiland said permanent magnet interact to create pivotal movement of saidfunctional surface at said resonant frequency.
 10. The multilayereddevice of claim 9 wherein said selected sweep frequency is substantiallyequal, to the resonant pivoting frequency of said device about saidfirst axis.
 11. The multilayered device of claim 1 wherein said backlayer has a size and shape substantially matching said size and shape ofsaid attaching member.
 12. The multilayered device of claim 11 andfurther comprising piezoelectric material bonded to said supportstructure of said hinge layer to create resonant pivoting of saiddevice.
 13. The multilayered device of claim 1 wherein said hinge layeris made from single crystal silicon.
 14. The multilayered device ofclaim 13 wherein said front layer is made from single crystal silicon.15. The multilayered device of claim 5 wherein said back layer is apermanent magnet.
 16. The multilayered device of claim 15 and furthercomprising a magnetic coil connected to an alternating voltage having afrequency equal to a selected sweep frequency of said pivoting deviceand wherein said magnetic coil and said permanent magnet interact tocreate pivotal oscillations of said device at said selected sweepfrequency.
 17. The multilayered device of claim 16 wherein said selectedsweep frequency is substantially equal, to the resonant pivotingfrequency of said device.
 18. The multilayered device of claim 5 whereinsaid back layer has a size and shape substantially matching said sizeand shape of said attaching member.
 19. The multilayered device of claim18 and further comprising piezoelectric material bonded to said supportstructure of said hinge layer to create resonant pivoting of said deviceabout said first axis.
 20. The multilayered device of claim 5 whereinsaid hinge layer is made from single crystal silicon.
 21. Themultilayered device of claim 20 wherein said front layer is made fromsingle crystal silicon.
 22. The multilayered device of claim 1 whereinsaid functional surface of said front layer is a mirror.
 23. Themultilayered device of claim 22 wherein said back layer is a permanentmagnet.
 24. The multilayered device of claim 23 and further comprising amagnetic coil connected to an alternating voltage having a frequencyequal to a selected sweep frequency of said mirror functional surfaceand wherein said magnetic coil and said permanent magnet interact tocreate pivotal oscillations of said mirror at said selected sweepfrequency.
 25. The multilayered device of claim 24 wherein said selectedsweep frequency is substantially equal to the resonant frequency of saidmirror functional surface about said first axis.
 26. The multilayereddevice of claim 25 used as a drive engine to provide a scanning beam.27. The multilayered device of claim 26 wherein said drive engineprovides the scanning beam for a printer.
 28. The multilayered device ofclaim 22 wherein said back layer has a size and shape substantiallymatching said size and shape of said attaching member.
 29. Themultilayered device of claim 28 and further comprising piezoelectricmaterial bonded to said support structure of said hinge layer to createresonant pivoting of said device.
 30. The multilayered device of claim29 used as the drive engine to provide a scanning beam.
 31. Themultilayered device of claim 5 wherein said functional surface of saidfront layer is a mirror.
 32. The multilayered device of claim 31 andfurther comprising a magnetic coil connected to an alternating voltagehaving a frequency equal to a selected sweep frequency of said pivotingdevice and wherein said magnetic coil and said permanent magnet interactto create pivotal oscillations of said device at said selected sweepfrequency.
 33. The multilayered device of claim 32 wherein said selectedsweep frequency is substantially equal, to the resonant pivotingfrequency of said device.
 34. The multilayered device of claim 31wherein said back layer has a size and shape substantially matching saidsize and shape of said attaching member.
 35. The multilayered device ofclaim 34 and further comprising piezoelectric material bonded to saidsupport structure of said hinge layer to create resonant pivoting ofsaid device about said first axis.
 36. The multilayered device of claim31 wherein said hinge layer is made from single crystal silicon.
 37. Themultilayered device of claim 36 wherein said front layer is made fromsingle crystal silicon.